1. Field of the Invention
This invention relates generally to filter elements, and is concerned in particular with an improvement in the design and manufacture of green baskets for use in removing contaminants from papermaking stock.
2. Description of the Prior Art
U.S. Pat. No. 3,631,981 (Young) discloses one known method of manufacturing screen baskets by first cutting mutually aligned and communicating screening and relief slots into opposite faces of a plate, after which the plate is rolled into a cylinder and reinforced by circular ribs or bands. The screening slots are continuous, but the relief slots are interrupted by lands and are thus discontinuous. A disadvantage of this arrangement is that the lands create blockages in the screening slots, thereby reducing filtering capacity while also providing sites at which solids can snag and begin to accumulate. These problems are exacerbated by the arcuate extensions at the ends of the lands which result from "plunge cutting" the relief slots with circular saw blades. The discontinuous relief slots aim weaken the screen basket structure by creating sites for fatigue failure due to higher stress concentrations at the slot ends.
Efforts have been made to remove the arcuate extensions at the ends of the lands by resorting to further manufacturing operations, but the additional expense of doing so impacts unfavorably on the overall cost of the resulting product. Moreover, the structural integrity of the screen basket can be compromised by these additional manufacturing operations, thus necessitating the incorporation of additional reinforcement.
Other cutting methods involving the use of laser beams, electron beams, water jets, etc. do not produce arcuate extensions at the ends of the lands. However, the lands are still required in order to maintain structural integrity during manufacturing. Thus, screening slot blockage remains a problem, even with slotted screen baskets produced by these other cutting methods.
As disclosed in the U.S. Pat. Nos. 3,716,144 (Bartlow) and 5,255,790 (Einoder et al), it is also known to fabricate screens from a plurality of wires secured by welding or brazing to transversely extending support elements. The wires are profiled and suitably spaced to define continuous screening slots communicating with continuous relief slots, the latter being traversed at spaced locations by the support elements. This arrangement has the advantage of providing screening slot continuity. However, this advantage is largely offset by the non-uniform slot widths which result from the difficulty of maintaining a constant precise spacing between the wires. This difficulty is compounded by the thermal distortions that almost invariably accompany welding or brazing operations.